It is known that in copying and printing machines of the electrophotographic type, an electrostatic latent image holder, generally consisting of a conductive cylinder coated with a layer of photoconductive material, is juxtaposed, along a generatrix of the cylinder, to a developing material carrier which is also generally cylindrically-shaped. The two cylinders may be in contact or spaced apart with a predetermined gap therebetween, generally on the order of 50 to 500 microns. The two cylinders usually rotate in the opposite direction with the same peripheral speed. In some implementations, however, they rotate with differing peripheral speeds and/or in the same direction.
A thin layer of powder developing material, known as "toner", suitably electrized by triboelectric effect, is formed on the surface of the developing material carrier, hereinafter designated as the developing roller. The toner, which generically has magnetic properties, adheres to the developing roller due to magnetic fields suitably generated on the developing roller surface and to Van Der Waal forces which act between the toner granules and the developing roller in spite of an electrical potential applied to the developing roller relative to ground, of the same polarity as the electrical charge acquired by the toner due to the triboelectric effect. This charge is generally negative.
The conductive cylinder of the latent image carrier (which in the following will be designated as OPC due to the extensive use of Organic Photo Conductive materials for its implementation) is generally grounded. An electrical charge, generally negative, is formed on the OPC surface by means of an electrostatic charge generator. This electrical charge lowers the surface potential to a predetermined value, for example -680 v referenced to ground.
The OPC generatrixes, duly electrized, reach, due to the OPC rotation, an exposing station where the OPC is selectively exposed to an electromagnetic radiation. In the exposed zones, the photoconductive material loses its electrical charge, and its electrical potential drops virtually to 0 v (in practice, to about 50 v). The several OPC generatrixes so exposed then reach the developing station where the toner particles, negatively charged and immersed in the electrical field formed by the differing potential of the developing roller and the OPC, are attracted onto the OPC in the OPC zones where it has been discharged to 0 v.
In the zones where the OPC remains charged (-680 v) the electrical field opposes the toner migration from the developing roller. Continuing its rotation, the OPC carries the toner particles, selectively located on its surface, into a transfer zone or transfer station where the OPC contacts, along one of its generatrixes, a printing support (generally a paper sheet) which is fed with the same speed as the OPC.
In the transfer station, the printing support is interposed between OPC and an electrostatic charge generator which charges the printing support with a positive charge. The positive polarization is sufficient to attract the toner from the OPC to the printing support where the toner adheres and is subsequently permanently fixed in a fixing station.
This process, conceptually very simple, does not produce perfect results in practice corresponding to the desired ones. The toner transfer from the developing roller to the OPC occurs not only in the zones where it is required, but to some extent also in the zones where it is not desired providing a "background" effect which hampers the quality of the images which can be obtained. This is due both to the impossibility of obtaining a sharp change of the electrical field at the borders of the latent image and to the impossibility of uniformly charging the several toner particles. It must be assumed that, statistically, a certain number of particles are weakly charged, not charged at all or electrically charged with the opposing polarity.
The adherence of electrically neutral particles to the OPC rather than to the toner carrying drum cannot be controlled by means of electrical fields and escapes control. Even in case of weakly charged particles, the control action exerted by the electrical fields is to some extent inadequate.
To overcome these limitations, it has been proposed (and described in several patents among which U.S. Pat. No. 3,866,574 is representative) to apply an a-c voltage to the developing roller, in addition to the biasing d-c voltage, in order to generate a pulsing electrical field between the OPC and the developing roller. Several frequencies and amplitudes have been proposed to achieve some enhancement of the printed images in terms of contrast, resolution and background reduction.
Basically two explanations have been given for these results. First, the pulsing electrical field should cause a vibration of the toner particles which makes easier their detachment from the developing roller even if the particles are weakly charged. Second, the pulsing field (at the extreme an alternating field) should cause a particle rebound from the OPC to the developing roller with the consequence of collisions among particles and detaching of a greater amount of toner from the developing roller.
Whatever the explanation may be, the results which have been achieved are limited. The present invention overcomes these limitations and provides a developing apparatus in which the background effect is minimized and the image resolution is enhanced to an extreme level. In addition, the efficiency of the process is improved, and the toner amount which is wasted is reduced to a minimum.